Current trends in the electronics industry are moving toward higher signal, power, and ground line densities, smaller-size and multiple-layered packages, and increased performance characteristics, such as less crosstalk, lower inductances, and greater resistance to failure from thermal cycling stress. As a result of the increased line densities, the density of interconnections increases and the surface area available for the interconnections, land sites, etc. is at a premium. For example, land areas in the order of 25 mils (0.64 cm) and interconnection hole diameters less than 10 mils (0.0254 cm) are considerably smaller than those in conventional printed circuit board applications. Interconnection holes are also referred to as through holes or vias.
An electronic material structure which is useful in such high density applications is a dielectric substrate composed of a polyimide polymer and a conductor layer, usually copper, on a single or both sides of the substrate. There are many methods by which holes in both the conductor and a dielectric layers can be formed. Typical methods for forming holes can be by punching, mechanical drilling, laser drilling, or photoimaging and etching with either chemical or plasma etching techniques. Not all these methods however are able to adequately form vias with diameters 6 mils (0.015 cm) or less in a polyimide substrate. Chemical etching of a polyimide substrate involves removal of the substrate with an etchant solution which comprises of ethyl or propyl alcohol or mixtures thereof with a basic solution and optionally with a diamine solution. A metal mask or the conductive pattern on the substrate acts as a mask to direct the etch removal of the substrate. There is a disadvantage to chemical etching in that the polyimide substrate may etch to a greater degree in the lateral direction (x-y plane) than in the vertical direction (z plane). This results in the substrate undercutting the conductor layer. Undercut in the formation of small diameter vias is a problem because it increases the size of the pad needed to cover the via. Vias 40 mils or greater in diameter can be accomplished easily with existing mechanical drills. However, mechanical drills less than 15 mils (0.038 cm) in diameter are extremely fragile and subject to breakage which requires time consuming and expensive replacement and repair.
One possible solution to the problem of drilling small diameter vias is the implementation of laser drilling. Several type of lasers, such as excimer and argon ion, are available to drill dielectric materials. The excimer laser is the only known method of laser drilling polyimide substrates to produce clean, small diameter holes in a single step. The excimer laser uses photoablation with electromagnetic energy in the ultraviolet region of the spectra to break the chemical bonds of the substrate and ablate the polyimide. However, an excimer laser is considered a laboratory instrument and as such it is slow and costly and is not readily adapted to an industrial or manufacturing environment. Laser drilling of a polyimide substrate can also be accomplished by use of an argon ion laser. In this method, the area where a hole in the substrate is desired is irradiated with an argon ion laser beam that has bursts of electromagnetic energy at a level sufficient to damage the film but without ablating it. The substrate is then plasma etched so as to remove the polyimide damaged by irradiation. Plasma etching uses a gas, usually oxygen, which attacks and removes the exposed substrate but not the conductor layer. Plasma etching must be closely controlled since it is highly dependent on the temperature of the etchant and gas pressure. The disadvantages to this method are that there is an additional step of plasma etching and that this step typically results in variant diameter holes or deviant hole shape, i.e., undercut.
Further, carbon dioxide lasers have been used to drill holes in epoxy-glass substrates, such as FR4, but not in polyimide substrates. The difficulty with carbon dioxide laser drilling of polyimide substrates is that there is a considerable amount of debris in and about the hole after irradiation. The debris must be cleaned away before any subsequent plating steps as the debris will interfere with electrical interconnections of the component. An additional step of etching by either plasma or chemical etching would need to be done to clean away the debris. As described above, etching by either method is undesirable since the variation in hole diameter or deviation in hole shape directly affects subsequent steps, i.e. plating, bonding, interconnection etc. resulting in poor component performance.
It is therefore an object of this invention to provide a method of forming a small diameter hole in a polyimide substrate by employment of a carbon dioxide laser which results in a hole free of debris and not substantially undercut.